How much time do you have for this itinerary?

MNL is always cheap to fly to although if you're not planning on any other destinations in the Philippines it might not be worth it to go there at all.

SGN should also have a number of cheap flights at any time, HAN less so.

Of the options you gave, I think the best value can be had on a BKK or SIN redemption but it might not be very worthwhile anyway considering there are lots of other options on these routes. Without a redemption, you could still get to Bangkok cheaply on VN with SGN as a stopover or flying to Bangkok's other airport, DMK, and if you decided to do Singapore, you could fly in or out of JHB, although I'd skip it and spend more time in any of the other places instead (particularly in Vietnam).

It could be more worth it to fly on points into some smaller airport instead.